Thyroid hormones, such as L-thyroxine (T4) and 3,5,3′-triiodo-L-thyronine (T3), and their analogs such as GC-1, DITPA, Tetrac and Triac, regulate many different physiological processes in different tissues in vertebrates. It was previously known that many of the actions of thyroid hormones are mediated by the thyroid hormone receptor (“TR”). A novel cell surface receptor for thyroid hormone (L-thyroxine, T4; T3) has been described on integrin αVβ3. The receptor is at or near the Arg-Gly-Asp (RGD) recognition site on the integrin. The αVβ3 receptor is not a homologue of the nuclear thyroid hormone receptor (TR), but activation of the cell surface receptor results in a number of nucleus-mediated events, including the recently-reported pro-angiogenic action of the hormone and fibroblast migration in vitro in the human dermal fibroblast monolayer model of wound-healing.
Tetraiodothyroacetic acid (tetrac) is a deaminated analogue of T4 that has no agonist activity at the integrin, but inhibits binding of T4 and T3 by the integrin and the pro-angiogenic action of agonist thyroid hormone analogues at αVβ3. Inhibition of the angiogenic action of thyroid hormone has been shown in the chick chorioallantoic membrane (CAM) model and in the vessel sprouting model involving human dermal microvascular endothelial cells (HDMEC). In the absence of thyroid hormone, tetrac blocks the angiogenic activity of basic fibroblast growth factor (bFGF, FGF2), vascular endothelial growth factor (VEGF) and other pro-angiogenic peptides. Tetrac is effective in the CAM and HDMEC models. This inhibitory action of tetrac is thought to reflect its influence on the RGD recognition site that is relevant to pro-angiogenic peptide action.
Evidence that thyroid hormone can act primarily outside the cell nucleus has come from studies of mitochondrial responses to T3 or T2, from rapid onset effects of the hormone at the cell membrane and from actions on cytoplasmic proteins. The recent description of a plasma membrane receptor for thyroid hormone on integrin αVβ3 has provided some insight into effects of the hormone on membrane ion pumps, such as the Na+/H+ antiporter, and has led to the description of interfaces between the membrane thyroid hormone receptor and nuclear events that underlie important cellular or tissue processes, such as angiogenesis and proliferation of certain tumor cells.
Circulating levels of thyroid hormone are relatively stable; therefore, membrane-initiated actions of thyroid hormone on neovascularization or on cell proliferation or on membrane ion channels—as well, of course, as gene expression effects of the hormone mediated by TR mentioned above—may be assumed to contribute to ‘basal activity’ or setpoints of these processes in intact organisms. The possible clinical utility of cellular events that are mediated by the membrane receptor for thyroid hormone may reside in inhibition of such effect(s) in the contexts of neovascularization or tumor cell growth. Indeed, we have shown that blocking the membrane receptor for iodothyronines with tetraiodothyroacetic acid (tetrac), a hormone-binding inhibitory analogue that has no agonist activity at the receptor, can arrest growth of glioma cells and of human breast cancer cells in vitro. Tetrac is a useful probe to screen for participation of the integrin receptor in actions of thyroid hormone.
Integrin αVβ3 binds thyroid hormone near the Arg-Gly-Asp (RGD) recognition site of the protein; the RGD site is involved in the protein-protein interactions linking the integrin to extracellular matrix (ECM) proteins such as vitronectin, fibronectin and laminin. Also initiated at the cell surface integrin receptor is the complex process of angiogenesis, monitored in either a standard chick blood vessel assay or with human endothelial cells in a sprouting assay. This hormone-dependent process requires MAPK activation and elaboration of basic fibroblast growth factor (bFGF; FGF2) that is the downstream mediator of thyroid hormone's effect on angiogenesis. Tetrac blocks this action of T4 and T3, as does RGD peptide and small molecules that mimic RGD peptide. It is possible that desirable neovascularization can be promoted with local application of thyroid hormone analogues, e.g., in wound-healing, or that undesirable angiogenesis, such as that which supports tumor growth, can be antagonized in part with tetrac.
Thyroid hormone can also stimulate the proliferation in vitro of certain tumor cell lines. Murine glioma cell lines have been shown to proliferate in response to physiological concentrations of T4 by a mechanism initiated at the integrin receptor and that is MAPK-dependent. In what may be a clinical corollary, a prospective study of patients with far advanced glioblastoma multiforme (GBM) in whom mild hypothyroidism was induced by propylthiouracil showed an important survival benefit over euthyroid control patients. We reported in 2004 that human breast cancer MCF-7 cells proliferated in response to T4 by a mechanism that was inhibited by tetrac. A recent retrospective clinical analysis by Cristofanilli et al. showed that hypothyroid women who developed breast cancer did so later in life than matched euthyroid controls and had less aggressive, smaller lesions at the time of diagnosis than controls. Thus, the trophic action of thyroid hormone on in vitro models of both brain tumor and breast cancer appears to have clinical support.
The cellular or tissue actions of thyroid hormone that are known to be initiated at integrin αVβ3 and that require transduction of the hormone signal via MAPK are summarized below. The integrin is a signal transducing protein connecting signals from extracellular matrix (ECM) proteins to the cell interior (outside-in) or from cytoplasm and intracellular organelles to ECM (inside-out). Binding of L-thyroxine (T4) or 3,5,3′-triiodo-L-thyronine (T3) to heterodimeric αVβ3 results in activation of mitogen-activated protein kinase (MAPK; ERK1/2). Activated MAPK (phosphoMAPK, pMAPK) translocates to the cell nucleus where it phosphorylates transactivator proteins such as thyroid hormone receptor-β1 (TRβ1), estrogen receptor-α (ERα) or signal transducer and activator of transcription-1α (STAT1α). Among the genes consequently transcribed are basic fibroblast growth factor (bFGF), that mediates thyroid hormone-induced angiogenesis) and other proliferation factors important to cell division of tumor cells.
There is thus a need in the art for thyroid hormone analogs that can bind to the cell surface receptor while not being able to enter the cell. Such reformulated hormone analogues would not express intracellular actions of the hormone and thus if absorbed into the circulation would not have systemic thyroid hormone analog actions.